Israel killed three armed Islamic activists as it continued a week-long assault against Hamas and its allies that Palestinians said was intended to undermine the groups in local elections held yesterday.

The army said it had arrested more than 420 "wanted Palestinians" during the past week in nearly 100 raids on West Bank towns, including Hebron, Nablus and Ramallah. Palestinian human rights groups said the detentions appeared, in part, to be a round up of candidates and political activists after Ariel Sharon said he would not accept Hamas running for election unless it disarmed and renounced its intention to destroy Israel.

The raids also targeted welfare groups associated with Hamas which have helped the organisation build popular political support. The army closed the offices of the Islamic Charitable Society in several villages and took away equipment.

But the military said the arrests in the West Bank were an attempt to end continuing rocket attacks from the Gaza Strip by Islamist groups, more than a fortnight after Israel withdrew its forces from the territory.

Israeli forces have also launched about 30 aerial attacks on Gaza, destroying schools and homes as well as firing artillery into open areas of Gaza as a "warning" for the first time.

Yesterday the army killed Samar Sa'adi, the head of the al-Aqsa Martyrs Brigade in Jenin, during a gun battle. Soldiers also shot dead two other men, identified as members of Islamic Jihad, in the village of Burqin. At the weekend Israeli forces killed four Hamas and Islamic Jihad activists in two aerial missile strikes on cars in the Gaza Strip.

Mahmoud Abbas, the Palestinian leader, urged the US to intervene to "take action to stop these operations that could provoke a collapse of the situation". A meeting between Mr Abbas and Mr Sharon scheduled for next week has been called off because of the violence.

The Palestinian police said yesterday they had begun enforcing a ban on carrying weapons in public in an attempt to curb the activities of the armed groups. But this did not prevent Palestinians from firing two mortars into an army base near the Gaza Strip, without causing injury. Shaul Mofaz, the defence minister, said Israel would keep up its strikes until the rocket attacks stopped.

The Israeli action was prompted by Hamas firing rockets into Israel in an attempt to falsely blame it for an explosion at an Islamist rally that killed 17 people a week ago. The Palestinian Authority said Hamas was responsible for this and should apologise.

Mr Sharon appears to have taken the opportunity to launch a broad assault against Hamas after warning that Israel would never accept the group as part of the Palestinian political process while it remained armed and hostile.

The Israeli action has shaken Hamas in part because many ordinary residents of Gaza, who had hoped for a relatively peaceful existence after the Israeli pullout, are blaming the group for the upsurge in violence. In response, its political leader in the occupied territories, Mahmoud al-Zahar, declared an end to attacks from the Gaza Strip and dropped his usually fiery threats to continue the war against Israel.

It will only become clear in the coming days if the attacks damaged Hamas's standing in yesterday's local election for more than 1,000 council seats in the Palestinian territories, the biggest test of the group's support before January's ballot for the Palestinian parliament. Opinion polls give Hamas up to a third of the vote at the expense of Mr Abbas's Fatah faction.

Hamas further infuriated the Israeli government by releasing a video of a businessman who was shot dead after pleading for the release of Palestinian prisoners. The group said it had intended to hold the murdered man, Sasson Nuriel, as a hostage but killed him in response to the detention of its activists.

Amid criticism from the Israeli right, which has blamed the violence on the Gaza pullout, Mr Sharon said yesterday there would be no further unilateral withdrawals from the occupied territories.

The prime minister contradicted comments by one of his senior advisers, Eyal Arad, who said that Israel might unilaterally impose the borders of a Palestinian state. Mr Arad was implicitly backed by the head of Israel's military intelligence, Aharon Ze'evi, in a speech to a security conference. "In the coming years, Israel will have to take more and more unilateral steps, in order to advance its own interests," he said. But Mr Sharon said Israel was committed to the US-led "road map" which envisages a negotiated settlement leading to a Palestinian state.